The transmission of pulses of ultrasonic energy into an object and using the reflected echoes from acoustic discontinuities within the object to create a cross-sectional image of the object is a well-known technique in medical diagnosis and in non-destructive evaluation. In one application of this technique, called "compound scan imaging", the pulses of ultrasonic energy are directed into the object from a number of inspection positions and an intersecting pattern of lines of sight are established to provide a complete cross-sectional image of the object. Using compound scan imaging, a panoramic view of an entire cross-section of the object under investigation can be obtained. Areas are scanned from a number of directions, which provides good coverage of echoes from specularly reflecting surfaces and avoids problems due to local areas of shadowing, which would otherwise degrade areas of the image.
A disadvantage of the compound scan approach to ultrasonic imaging is that it usually requires a time period of several seconds to obtain an image. Any movement of the scanned object during this period causes distortion or blurring of the resultant image. Also, variation of the speed of sound in the various regions of the object tends to cause mis-mapping of echo position, and this, with overlapping scans, causes further image degradation.
In a relatively recent development in medical ultrasonic imaging, a single inspection point is used and a beam of ultrasonic energy is rapidly scanned to produce a sector of ultrasonic echo information in a fraction of a second. This process is repeated continuously and produces a continuously refreshed ultrasonic image which depicts the anatomy under investigation in cross-section. This technique is particularly useful for examining moving objects, such as the beating heart or the foetus, and is also useful for carrying out a rapid survey of the anatomy to establish which areas require closer examination.
The available frame rate (or rate of refreshment) of an ultrasound image is determined by the depth of penetration required, the speed of sound in the object (in human tissue this has an average value of 1540 meters per second), and the number of lines necessary to make a satisfactory image. For medical ultrasound imaging, the frame rate R is given by the relationship: ##EQU1## where N is the number of lines required and d is the required depth in cm.
The number of lines needed for a satisfactory image is set by the maximum angular distance between successive lines (which in turn is set by the beam width) and the total angle of the sector scan, provided the angular speed of scan generation is constant. In a typical case of 128 scan lines, the maximum frame rate is 30 frames per second.
One method used to implement real-time sector scanning is to cause one or a plurality of transducers to move so as to generate a sector scan pattern. This approach is called "mechanical sector scanning". An alternative method which has found some application uses a transducer array with electronic steering of the beam.
A common method used to achieve a sector scan mechanically is to mount a number of transducers on the side of a barrel-shaped member which is mounted in a housing having an acoustic window. When the barrel-shaped member is rotated about its axis, each transducer in turn is presented to the window and is activated while it rotates before the window. Thus, each transducer generates a sector scan of ultrasonic echo lines of sight. As the active transducer passes away from the window, the next transducer enters the window area and becomes active. This method, known as the "spinner" technique, has the advantage that, because it involves continuous rotation of a balanced member, vibration is not a problem. Also, the angular speed is constant, allowing a constant angular line spacing and the largest possible frame rate.
Another method which has been used to achieve a sector scan mechanically is to use a single transducer which is mounted to oscillate about an axis which passes through the transducer. The mounting is driven by a crank to produce an angular oscillatory motion of the transducer. Such an oscillating transducer is known as a "wobbler". As this is the starting point for the improvement in real-time scanning which is achieved with the present invention, it will be discussed in some detail.
To avoid high angular accelerations, the oscillatory motion of the "wobbler" transducer is faster in the centre of the scan and slower towards the edges, where the scan direction reverses. As the pulse repetition rate of the ultrasound transmit pulses is constant with time, the spacing of scan lines in the image generated using this type of equipment is greatest at the centre of the scan, where the angular speed is greatest, and is least near the edges of the scan as the angular speed is reduced. As the scan rate is set by the maximum angular distance between scan lines, the angular speed at the middle of an oscillatory scan is the same as that of the constant angular speed of a spinner scanner. Since the lines of sight of the ultrasonic pulses in other parts of the scan are more crowded, and the scan time is proportional to the number of lines, the presence of the extra lines means that more time is required for a scan. In fact, the total scan time is greater by about a factor of two when compared with the spinner technique. This consideration leads to a maximum frame rate for an oscillatory scan being about half that for a constant angular speed spinner scan.
This oscillatory or "wobbler" approach has benefits in that the equipment used can be made lighter, and a smaller coupling area to the body is required. For these reasons, it has been advantageous to employ this approach in many areas of clinical examination.
There is another advantage of the "wobbler" approach which is related to the method commonly used to interpolate the image content between the ultrasound data lines. The most economical interpolation method is to interpolate linearly along horizontal lines in the image, as this can be done during the display of the individual scan lines. The properties of the ultrasonic image are such that the most appropriate way to interpolate is at right angles to the scan line, as described more fully in a paper by D E Robinson and P C Knight entitled "Interpolation Scan Conversion in pulse-Echo Ultrasound", which was published in Ultrasonic Imaging, Volume 4, pages 297-310, 1982. For parts of the image near the centre of the scan, the sector scan lines are at right angles to the horizontal raster lines and the interpolation is appropriate. Towards the edges of the scan, the ultrasound lines are inclined and horizontal interpolation becomes less appropriate, but this can be overcome by reducing the angular spacing of the data lines, which is a property of the oscillating wobbler mechanical scan.
With the improvement in image quality available in real-time scanners, the older compound scan technique fell out of favour. However, there are a number of specific advantages in the compound scan technique which make a combination of real-time scanning and compound scanning attractive. These advantages arise from the need to carry out specific functions which are additional to the imaging. One of these specific functions is the measurement of fluid flow in vessels using the observed Doppler shift of echo signals in conjunction with the determination of the cross-sectional area of the vessel and angle between the axis of the vessel and the incident beam of ultrasonic energy. An example of the use of the Doppler effect was described by G Kossoff in the specification of Australian Pat. No. 492,512, which corresponds to U.S. Pat. No 3,939,707, U.K. Pat. No. 1,459.849 and Japanese patent application No. 54311/74. The use of at least two transducer positions is needed for the determination of sound speed within examined tissue, as was shown by D E Robinson in the specification of Australian Pat. No. 523,895 (which corresponds to U.S. Pat. No. 4,252,025 ), and by D E Robinson, C F Chen and L S Wilson in their paper entitled "Measurement of Velocity of Propagation from Ultrasonic Pulse-Echo Data", which was published in Ultrasound in Medicine and Biology, volume 8, No. 4, pages 413-420, 1982.
It is an obvious progression from these examples of the prior art to attach two real-time mechanical sector scanners to an arm and provide appropriate display means to combine the spatially related images from the two transducer positions, which is well known from compound scanning. In this straight-forward application of two "wobbler" real-time sector scanners, the maximum frame rate for a full sector scan from each transducer would be just half that for each sector alone. However, this is undesirable as it gives rise to an annoying flicker in the image and blurring and jerkiness of the images of moving structures.